Method and apparatus for real-time measurement of trace metal concentration in chemical mechanical polishing (cmp) slurry

ABSTRACT

A system (and method) for real-time measurement of trace metal concentration in a chemical mechanical polishing (CMP) slurry, includes an electromagnetic radiation flow cell carrying a CMP slurry, a slurry pickup head coupled to the flow cell, and an analyzer for measuring properties of the slurry flowing through the flow cell.

The present application is a Divisional Application of U.S. patentapplication Ser. No. 10/953,380 filed on Sep. 30, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus formeasuring a material in a slurry during a polish process, and moreparticularly to a method and apparatus for accurately monitoring polishprocesses by measuring a removed metal concentration in the slurry.

2. Description of the Related Art

Copper and Tantalum liner polish operations typical of current copperback end of line (BEOL) (and aluminum BEOL in some cases) interconnectlayers involve the bulk removal of metal by chemical mechanicalpolishing (CMP) on processed semiconductor wafers.

In operation, there is some level of interest to which the designerwishes to make connections. To do so, an oxide is deposited over thelevel, lines and holes (vias) are made in the oxide, patterned thereinand extending down to the level of interest (e.g., the layer to whichthe connections are to be made). Then, a liner (e.g., formed oftungsten, tantalum, etc.) is formed over the wafer surface and in eachof the holes, and then a copper layer is electroplated over thestructure. Thereafter, the copper layer is polished back until the lineris exposed everywhere except where there are trenches or holes. Thus,all that is left is the interconnect wires, and the bulk copper isremoved.

A condition arises in that, as the final stage of polishing is reached,the copper concentration in the slurry drops, as the relatively thinliner (which has a possibly different selectivity to the polish) isreached, and thus the liner becomes present in the slurry (e.g., becausethe liner is now being ground). The liner components in the slurrytypically exist in fairly low concentrations (e.g., about 30 ppm toabout 50 ppm). Thus, detection is somewhat difficult.

It is desirable at this point to stop the copper polish before a largeamount of liner has been removed. Prior to the invention, no method hasexisted which could precisely stop the polishing process to avoidpolishing the liner.

Thus, the correct performance of these process operations depend on eachpolishing step being stopped precisely as the material of each layer isdepleted.

Prior to the present invention, no conventional method has beendeveloped which can achieve such precise stopping.

Further, it is noted that thermal endpoint methods exist which takeadvantage of the evolved heat due to friction. As the liner is reached,the friction between the polish pad and wafer in the presence of slurrychanges. This results in a sensible change in temperature in some polishoperations. Due to the thermal masses involved and the thermal timeconstant of the system, it takes a certain amount of time to sense thechange (typically several seconds or more). Thus, the thermal endpointmethod does not work for all processes.

SUMMARY OF THE INVENTION

In view of the foregoing and other exemplary problems, drawbacks, anddisadvantages of the conventional methods and structures, an exemplaryfeature of the present invention is to provide a method and structure inwhich real-time measurement of predetermined trace material (e.g.,metal) concentration in a chemical mechanical polishing (CMP) slurry canbe performed.

In a first aspect of the present invention, a system (and method) forreal-time measurement of trace metal concentration in a chemicalmechanical polishing (CMP) slurry, includes an electromagnetic radiationflow cell for carrying a CMP slurry, a slurry pickup head coupled to theflow cell, and an analyzer for measuring the concentration of a materialof interest in the slurry flowing through the flow cell.

With the unique and unobvious method and structure of the invention, thewaste slurry is pumped from a pickup head on a platen (or directly froma platen drain) through a flow cell where a concentration of a desiredmetal is measured in real-time using, in a preferred example, x-rayfluorescence methods. The flow cell is sensitive to concentrations inthe ˜30 ppm range, and responds on the order of seconds or tens ofseconds.

Thus, with the invention, as a given layer of surface metal is depletedor encountered by CMP polishing, the concentration of metal monitoredwill change. Sensing this change enables the process endpoint to bedetected and the process monitored.

Additionally, process monitoring advantages include the feedback of theactual top surface metal thickness to prior level deposition processesand the uniformity of thickness across the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other exemplary purposes, aspects and advantages willbe better understood from the following detailed description of anexemplary embodiment of the invention with reference to the drawings, inwhich:

FIG. 1 illustrates a flow diagram of a system 100 according to thepresent invention;

FIG. 2 illustrates a system block diagram of a slurry trace elementanalyzer 200 according to the present invention;

FIG. 3 illustrates a slurry spectra 300 of the slurry trace elementanalyzer according to the present invention;

FIGS. 4A-4C illustrate wafer surface films during CMP copper and linerpolishing operations according to the present invention, and morespecifically:

FIG. 4A illustrates a side view of a wafer surface before copperpolishing;

FIG. 4B illustrates a side view of the wafer surface after copperpolishing and before liner polishing; and

FIG. 4C illustrates a side view of the wafer surface after linerpolishing; and

FIG. 5 illustrates a flowchart of a method 500 according to the presentinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1-5, thereare shown exemplary embodiments of the method and structures accordingto the present invention.

Generally, as discussed briefly above, the present invention allowssampling of the slurry immediately (and directly) coming off of thepolishing pad. Then, the sample is pumped through a flow cell in whichthe invention has constructed a very sensitive X-ray detector. The X-raydetector is for detecting the fluorescent X rays from a material ofinterest (e.g., copper, tantalum, titanium, aluminum, tungsten, etc.;any metal of interest). Each of these elements has a unique,characteristic fluorescent X-ray emission.

Thus, if these elements are bombarded with X rays, then the elementswill fluoresce, and will emit a characteristic X ray at a certainwavelength that is unique. Such a unique wavelength can be used toidentify the particular material in the presence of other differentmaterials. Ideally, a system should be provided which provides a fairlyrapid response. That is, polishing operations typically occur in tens ofseconds, or fractions of minutes. Thus, a system is desirable whichprovides a rapid, crisp response in seconds or less.

With X-ray operations, detection essentially requires counting.Conventionally, X-ray detectors in general are typically limited, due toelectronic considerations, etc., to a rate of about 50,000counts/second. Typically, an “accurate” count (e.g., a count made withcertainty that one knows the number) is proportional approximately tothe square root of the total number of counts.

Thus, if one wishes to be accurate to within 1%, then 10,000 counts mustbe obtained. Hence, in the optimal case, the invention excludes allphotons but those in the energy range of interest for that element.

For example, assuming a 10 keV emission line of interest, only thephotons at 10 keV are desired to be counted. Hence, the sample is beingilluminated with this broad sample of X rays. As a result, all of theelements in the sample are fluorescing, not just the ones of interest.If one were to simply position a counter near the sample, then all ofthe counts from all of the elements would be present. This isproblematic since only a fraction of a percent of the counts is from theelement of interest. Hence, if all that is done is to count the X rayscoming directly off of the sample after illuminating it, then to obtainan accurate count (given 50,000 counts/second) would take aprohibitively long time.

An exemplary embodiment of the present invention solves the above andother problems by integrating a bent crystal diffraction grating intothe design. The diffraction grating spatially spreads a partiallycollimated beam of photons according to their energy (similarly to aprism spreading a beam of light). By partially collimating thefluorescent emissions coming from the flow cell, directing theseemissions into the diffraction grating and into an X-ray detector placedto receive the photos of the desired energy, all but the desiredcharacteristic fluorescent emissions are detected.

Generally, a SiLi detector is used in combination with a multichannelanalyzer. The multichannel analyzer is understood to include a computerand electronics for pulse shaping, analog digital conversion, channelcounting and binning etc., and the control and analysis softwarenecessary to reduce the raw detector signal to a usable output signal(material concentration and endpoint). This configuration combineswavelength dispersive and energy dispersive methods of X-rayspectroscopy to achieve the desired result.

In the case where speed is less critical, the bent crystal can beexcluded and a detector with multichannel analysis capability used byitself.

That is, invention pumps waste slurry from a pickup head on a platen ordirectly from a platen drain through a flow cell where a concentrationof a desired metal is measured in real time by measuring its X-rayfluorescence. The flow cell is sensitive to concentrations in the ˜30ppm range, and responds on the order of tens of seconds or seconds.

Thus, with the invention, as a given layer of surface metal is beingdepleted by CMP polishing, the concentration of metal monitored willdrop. Sensing this allows precisely stopping the polishing operations.

Exemplary Embodiment

FIG. 1 illustrates a flow diagram of an apparatus 100 according to thepresent invention.

Specifically, slurry is removed from a polishing platen 110 (having apolishing pad 115) using a pickup head 120 or a direct connection to theplaten drain. A carrier 125 is adjacent the slurry pickup head 120.Then, the slurry is piped through a flow cell contained in the traceelement analyzer 140 shown in FIG. 1 containing a thin-walled plastictube that is transparent to X rays 230.

A radiation source 225 (e.g., X ray source 225 not shown in FIG. 1, butillustrated in FIG. 2) illuminates the tube (slurry line 230). The X-raysource is set to produce X rays up to 30 keV. The X-ray photonstraveling through the tube produce fluorescence in the metal atomscontained in the slurry.

The fluorescent X rays, produced in the flow tube 230, illuminate a bentcrystal illuminate diffraction grating (shown in greater detail in FIG.2 at reference numeral 235), and are diffracted from there to an energydispersive X-ray detector (shown in greater detail in FIG. 2 atreference numeral 240). Counts from the detector are monitored andinterpreted by a computer (microprocessor) 250 which also connects tothe polisher to provide the signal to notify it when endpoint isreached. In the exemplary, non-limiting embodiment, a SiLi detector isused. Other detectors could be used includingscintillation/photomultiplier detectors, etc. Then, the slurry exits toa drain or a recirculation port.

The pickup head provides pumping action under normal circumstances ofplaten rotation and where the sensor is located below the level of theplaten due to the Bernoulli effect. Optionally, the system may include apump 130 between the pickup head 120 and the trace element analyzer 140to ensure precise flow or more convenient placement of the analyzer.

It is important to the function of the device that it be able to respondwith a time constant on the order of seconds (or preferably tenths ofseconds) As mentioned above, typical energy dispersive X-ray detectorwill count a maximum of 50,000 counts per second. To achieve a desiredresponse time, approximately 100 to 1000 counts per second should bemeasured in the region of interest for an accurate count.

FIG. 2 illustrates a system diagram of a slurry trace element analyzer200 (e.g., similar to that of analyzer 140 in FIG. 1) according to thepresent invention.

Again, slurry is removed from a polishing platen (having a polishingpad) using a pickup head or a direct connection to the platen drain.Then, the slurry is piped through a flow cell 230 (e.g., a slurry flowtube) containing a thin-walled plastic tube that is transparent to Xrays.

An X-ray source 225 illuminates the tube 230. Again, the X-ray source225 is set to produce X rays up to 30 keV. The X-ray photons travelingthrough the tube 230 produce fluorescence in the metal atoms containedin the slurry.

A detector shield 231 is positioned adjacent the flow tube 230 forshielding a detector 240. The shield 231 may be comprised of sheet metalor like material having suitable stopping power for the energy range ofthe source and is shaped to exclude X-ray scatter for all but thedesired path. It is noted that this is understood to include aperturesand slits at multiple points in the beam path.

The fluorescent X rays so produced in the flow tube 230 are routed to abent crystal diffraction grating 235. Many gratings can be used. Theparticular grating is chosen to diffract photons at the particular X-rayline energy at the correct angle to the detector and exclude adjacentand unwanted X-ray lines.

From the grating 235, X rays are diffracted to the detector 240 (e.g.,an energy dispersive X-ray detector such as a SiLi detector or the like)which is understood to contain an entrance aperture or slit.

The detector 240 is operatively coupled to a microprocessor 250 with theappropriate electronics for pulse shaping, analog-to-digital conversion,multichannel analysis, and control and signal interpretation softwarenecessary to output slurry concentration data 251 and/or an endpointsignal 252. In a different embodiment, the source 225, grating 235, anddetector 240 are replaced by visible wavelength optics to detect achange in color, and thus could function similarly to a spectrometer orthe like. Indeed, the slurry containing some of the trace elements(e.g., metals) may have a completely different color (e.g., be visiblydifferent) from a slurry which does not, even if the concentration isvery low.

Fluorescent X rays at many energies are produced in the flow tube 230.FIG. 3 shows such a spectrum with, for example, the tantalum linesshown.

That is, FIG. 3 shows the counts in relation to the multichannelanalyzer (MCA) channel. In FIG. 3, a raw X-ray fluorescence spectra forslurry with and without a Ta liner. The X-ray source was at about 20.0kV at a current of about 67 μA.

The Ta peaks (eV)(for L1/L2 spectroscopic levels) are 11681.5 and11136.1, respectively, and are shown at reference numerals 310 and 315.The Ta peak for L3 was 9881.1 and is shown at reference numeral 320. Thewaveform for slurry with the Ta liner is shown at reference numeral 330,whereas the waveform for slurry only is shown at reference numeral 340.

In this example, assuming the invention is interested in K line and Lline emissions characteristic of the element of interest, the inventionemploys the bent crystal analyzer 235 to achieve this selection.

That is, by adjusting the angle of the crystal 235, only photons withthe energy of interest are directed to the detector 240, while othersare blocked by an optional baffle 260.

The baffle 260 could be a slit or an aperture allowing certain photonsto be passed therethrough to the detector, but which blocks all otherphotons. This allows the invention to utilize most of the detectorcounting bandwidth to count photons of interest. Again, the baffle/slitis optional, as it is noted that the tube size also effectively definesthe spot size.

The counts per unit time at the fluorescent energy is proportional tothe concentration of the element. By counting the number of photons atthe energy of interest per unit time, the invention can monitor theconcentration of the desired material sufficiently for endpointdetection purposes. This data may be constructed by a computer system(e.g., microprocessor 250) attached to the X-ray detector 240.

Thus, the invention only counts the photons in the wavelength range ofinterest, and thus if 10,000 counts are needed, then, in principle, onecan obtain five measurements per second (assuming that there are enoughphotons to do it), which is purely a function of how bright the sourceis. Hence, a brighter source could be provided. Thus, the inventionprovides an efficient instrument which counts only the photons in thewavelength of interest, and such can be detected.

The effect of the CMP polishing process on the surface films of a waferis exemplarily shown in FIGS. 4A-4C for the copper 420 and copper liner415 CMP polish process. A prior level material 410 is provided, havingthe liner 415 formed thereon, followed by the copper layer 420deposition.

It is noted that as each layer of copper 420 or liner (e.g., Ta, etc.)415 is removed, there will be an abrupt change in the amount of surfacemetal flowing into the slurry as the polish reaches the interfacebetween the material being polished and the next material. The resultantchange in concentration of this surface metal in the slurry is what theinventive sensor monitors.

FIG. 5 illustrates a method 500 according to the present invention forreal-time measurement or trace metal concentration in a CMP slurry.

In step 510, slurry is removed from the polishing platen using thepickup head or a direct connection to the platen drain.

In step 520, the slurry is piped through the flow cell containing a thinwalled plastic tube that is transparent to x-rays

In step 530, the slurry in the tube is illuminated by a radiation source(e.g., an X-ray source will be assumed in the present exemplaryembodiment). The x-ray photons traveling through the tube producefluorescence in the metal atoms contained in the slurry.

In step 540, the fluorescent X rays produced in the flow tube are routedto a bent crystal diffraction grating, and from the grating to theenergy dispersive X-ray detector.

In step 550, the microprocessor outputs a slurry concentration dataand/or an endpoint signal.

As described above, the method of the invention allows the user to knowthe progress of the CMP process, and allows the user to control theprocess. Thus, for example, the invention allows stopping the CMP (e.g.,grinding) process immediately (e.g., through feedback), or could allowsome over-polish to occur.

Additionally, the invention is advantageous in that it allows theequipment of the system to be monitored. For example, if the slurryfailed (e.g., wrong slurry in the tool, the slurry was not beingdelivered in the correct rate, a defective pad, etc.; each of which maybe a “silent” failure), then the invention would detect the same. Thus,it would become obvious that the tool failed, thereby allowing thesystem to be shut down. As such, the invention provides a toolperformance monitor. Hence, if one tool is finishing (or starting) at adifferent time than the other tools in the system (e.g., one tool isoperable 5 seconds longer than others), the system can be shut down andthe failure investigated/remedied.

As described above, with the unique and unobvious method and structureof the invention, the waste slurry is pumped from a pickup head on aplaten or directly from a platen drain through a flow cell where aconcentration of a desired metal is measured in real-time using x-rayfluorescence methods. The flow cell is sensitive to concentrations inthe ˜30 ppm range, and can respond on the order of seconds (or tenths ofseconds).

Hence, with the invention, as a given layer of surface metal is depletedby polishing (e.g., CMP polishing), the concentration of metal monitoredwill drop. Sensing this drop will enable the process endpoint to bedetected.

While the invention has been described in terms of several exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

For example, the invention is not limited to X rays, but other radiationcould be used. Thus, for example, a system for real-time measurement oftrace metal concentration in a chemical mechanical polishing (CMP)slurry could be provided in which an optical source would be providedinstead of the X-ray source, an optical flow cell for carrying a CMPslurry would be provided instead of an X-ray flow cell, a diffractiongrating would be provided instead of the bent crystal analyzer, and aphotodetector would be provided instead of a SiLi detector. Thus, in theabove system, an optical spectrometer would be provided in place of theX-ray spectrometer.

Further, it is noted that Applicant's intent is to encompass equivalentsof all claim elements, even if amended later during prosecution.

1. A system, comprising: a flow cell analyzer including a detector fordetecting characteristics emission in one of optical and X-raywavelengths.